Self-baking electrode structure and method of operating same

ABSTRACT

Improved results in operating self-baking electrodes are achieved by constantly maintaining a bar made of an electrically conductive material well immersed in a carbon mass contained within a metal shell, measuring the voltage drop between the bar and the metal shell and measuring a ratio between this voltage drop and the electrode current.

United States Patent Inventor Marlo Cavigll Mestre (Venice), Italy Appl.No. 63,251 Filed Aug. 12, 1970 Patented Nov. 30, 1971 AssigneeMontecatlni Edison S.p.A. Milan, Italy Priority Aug. 22, 1969 Italy 21150 A/69 SELF-BAKING ELECTRODE STRUCTURE AND METHOD OF OPERATING SAME[50] Field of Search 13/18 [56] References Cited UNITED STATES PATENTS1,751,177 3/1930 Sem etal. 13/18 3,513,245 5/1970 Sullivan 13/18 XPrimary Examiner-Bernard A. Gilheany Assistant ExaminerR. N. Envall, Jr.AuomeyHubbell, Cohen & Stiefel ABSTRACT: improved results in operatingself-baking electrodes are achieved by constantly maintaining a bar madeof an electrically conductive material well immersed in a carbon masscontained within a metal shell, measuring the voltage drop between thebar and the metal shell and measuring a ratio between this voltage dropand the electrode current.

PATENTEUNUV30I97I 3,624,261

SHEET 2 [1F 4 HOUR INVENTOR M AR 10 (3 W IG-L'L PATENTED NUV30|97|13,624,261

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INvIsN'n m l/n jw [7 BY 1 v I I ATTORNE SELF -BAKING ELECTRODE STRUCTUREAND METHOD OF OPERATING SAME BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention relates to a method for operatingselfbaking electrodes. More particularly this invention relates to amethod for continuously measuring the baking degree of selfbakingelectrodes especially suitable for submerged arc furnaces. Thisinvention also relates to an apparatus for carrying out said method.

2. Description of the Prior Art Self-baking electrodes have been knownand used for a long time, for instance in electrical submerged arcfurnaces for producing metals, alloys, etc.

It is well known that a self-baking electrode substantially comprises ametallic, peripherally extending, vertical shell and a carbon masscontained therein. This metallic shell is generally provided with ametallic internal reinforcing structure. One of the purposes of thereinforcing structure is to support the weight of the carbon mass. Theelectrode is fed at its top end with a raw electrode-forming paste, madeup of pieces of calcined coal of various particle size mixed with abinder, usually pitch. As a result of the heat developed by the furnaceand of the joule heat due to the resistance encountered by the currentwhile flowing through the electrode (the electrode current), theelectrode-forming paste undergoes a process of gradual transformation.The carbon mass may be schematically subdivided, from the top downward,into four zones. Thus, in the first or top zone, where the temperatureis lower than about 100 C., the paste is in the solid state. In thesecond zone, wherein the temperature is generally between about l and300 C. (depending on the characteristics of the raw paste), the pastetakes on the characteristics of a liquid phase, the viscosity thereofgradually increasing downwardly. In this zone the paste may be termedmolten." In the third zone, wherein the temperature is generally betweenabout 300 and 700 C., the paste is in its baking stage. The tarry andpitchy substances decompose and distill; the electrode-forming pastethen gradually changes into a tough, compact carbon mass highly suitablefor carrying the electrode current.

In the fourth or lowest zone, where the temperature is in excess of 700C., the electrode is baked.

As the baked electrode is gradually lowered into the furnace in order tocompensate for its wear, the transformation process extends to newportions of the electrode; a new portion of molten paste enters thebaking stage and a new portion of solid paste goes over into the moltenstate.

It is also well known that metal conductors carrying the electriccurrent to the electrode must be fitted to the baked part of the carbonmass, that is, to the conducting portion of the self-baking electrode.

When, during the furnace operation, the electrode is periodicallylowered into the furnace, the electrical conductors must be shifted to anew electrode zone. This shifting of the electrode with respect to theelectric conductors (which is usually referred to as the slipping of theelectrode) is in general carried out periodically. The slipping, forinstance the daily slipping, of an electrode must of course compensatefor the wear out of the electrode during the same period of time. Theslipping frequency and the thereto related length of each slippingdepends on the baking degree of the electrode-forming paste, since onemust avoid having the unbaked or partially baked paste carry the currentas it is a poor electrical conductor. In such a case the electriccurrent would meet, in fact, a strong resistance to its flow and, if theelectrode current were not decreased (with a consequential loss ofpower) until normal baking conditions are restored, the excessive heatproduced by the' joule effect would cause a deterioration of theelectrode. Such deterioration may lead to the breakage of the electrodeitself. The measuring of the baking degree of an electrode has, up tonow, presented considerable difficulties.

In fact, one usually must rely on an uncertain evaluation by sight ofthe temperature of the electrode in the zone of the electric contacts.This evaluation of the temperature is, however, made difficult due tothe presence of the contacts themselves and becomes altogetherimpossible when other equipment screens off this zone, as is the case,for instance, in closed furnaces.

In practice the operator relies on a cautious procedure of frequent andlimited slipping of the electrodes, which involves, however, burdensomework and still does not eliminate the danger of breakages of theelectrode when the baking is at less than the nonnal degree.

SUMMARY OF THE INVENTION I have now found an improved method foroperating selfbaking electrodes, especially for those electrodes whichare particularly suitable for submerged arc furnaces. More particularly,I have found a method for continuously measuring the baking degree ofself-baking electrodes. I have also found an apparatus for carrying outsuch a method. In accordance with my invention, this method comprises:

a. constantly maintaining a bar made of an electrically conductivematerial inside the carbon mass of the electrode, the lower part of thisbar being immersed in the baked zone of the carbon mass so as to ensurea good and lasting electrical contact with that zone;

b. measuring the voltage drop between the conductive bar and the metalshell; and

c. measuring the ratio between the voltage drop and the electrodecurrent.

In fact, I have discovered that the value of the ratio thus obtained(hereinbelow referred to as index C" or more briefly C), is a measure ofthe baking degree of the electrode.

More particularly, the value of index C is inversely proportional to thebaking degree of the electrode inasmuch as high values of C correspondto low values of the baking degree and, thus, in practice to situationsin which the electrode should not be slipped and in which the currentdensity is or should be reduced.

In order of relationship, which defines an inverse proportionalitybetween C and the baking degree, has been chosen instead of a directproportionality because, at a constant electrode current, themeasurement of the index C may be simplified into a simple voltagemeasurement.

Thus, I have found that my invention provides a method which overcomesthe drawbacks and uncertainties of the above-mentioned empiricalevaluation methods of the prior art. I have, in fact, found that myinvention makes possible continuous and reliable information on thebaking degree of the electrode forming paste in a self-baking electrode,and thereby enables an operator to know how much the electrode may beslipped with respect to the electrical contacts without having todecrease the electrode current and therefore the output capacity of thefurnace without running the risk of breakages.

I havealso found that my invention allows a furnace operator tocontinuously monitor the baking process of the raw electrode-formingpaste (for instance, at the startup of a new furnace or after a longshutdown of a furnace) through the constant indication of the progressof the baking, up to the attainment of a normal baking degree.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustrationof a self-baking electrode and of an apparatus for carrying out themethod of the present invention.

abnormal, which may occur while operating a self-baking electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 indetail, there is shown a self-baking electrode device suitable forcarrying out the method in accordance with my invention.

A cylindrical metal shell I contains a carbon mass M consisting of anupper layer 2 of solid raw paste, a second layer 3 of molten paste, apaste baking zone 4 and section 5 of baked electrode. Thecurrent-carrying plates 6 and conductors 7 connect the electrode to theexternal electrical circuit not shown in FIG. 1. The carbon mass is ofthe type that is well known in the prior art. Specifically, it iscommonly prepared in the form of a paste made up of finely dividedparticles of calcined coal mixed with a binder, preferably pitch. Inthis state the carbon mass is what has been termed above alayer 2 ofsolid raw paste. Clearly, as the paste is subjected to the heat from thejoule effect of electric current passing through the electrode structureit is progressively converted from solid raw paste to molten paste andultimately to a baked electrode.

For the sake of simplicity, the well-known internal reinforcingstructure of the electrode has not been illustrated nor has the furnaceto which the electrode is applied. These are thoroughly conventional.

A metal bar 8 is disposed within the carbon mass M, preferably along thelongitudinal axis of the electrode throughout the length thereof. Inorder to maintain bar Sin its axial position inside the carbon mass M,tension rods (not shown in FIG. 1) made of an insulating material suchas wood or Bakelite may be employed to fix conductive bar 8 to thepreviously mentioned conventional reinforcing structure of theelectrode. The lower portion'of the conductive bar 8 is well immersed inthe baked zone 5 of the carbon mass so as to ensure a good electricalcontact with this zone.

In order to facilitate this electric contact, the bar material, forinstance a metal, is selected to have the highest possible melting pointthat is compatible with the requirement that it must be consumedtogether with the electrode. The choice of such a material, whichdepends on the type of process for which the electrode shall be used, iswell within the ability of the skilled art worker. In order tofacilitate electric contact between the bar and the carbon mass, theconductive bar is preferably shaped to offer a widesurface of contactwith the baked carbon mass, for instance in the shape of a strap orribbon or any other equivalent shape easily determined by the skilledart worker.

As already indicated, the conductive bar is intended to be consumedtogether with the carbon electrode and must be therefore periodicallyrestored by adding a new length of bar. This addition may be carried outsimply by welding a new length of bar to the upper end of the conductivebar, although other connecting procedures may be employed.

A device 9 for measuring the voltage drop between bar 8 and shell I, isconnected by means of conductors 10 and 11 to the upper ends of bar 8and shell 1.

The measuring of the voltage drop is preferably carried out betweenshell 1 and bar 8, inasmuch as when measuring between bar 8 and thecurrent carrying plates 6, the contact resistance between shell 1 andcurrent-carrying plates 6 would decrease the sensitivity of the method.To the same device 9, the measuring of the electrode current isconveyed, as by conductors I2 and 13.

The following Example is presented to further illustrate my invention.

EXAMPLE Reference is made to the apparatus of FIG. 1. In a threephaseelectrical furnace producing calcium carbide, the electrodes have adiameter of 950 mm. and were delta connected and supplied withalternating current through a delta-delta transformer. The electrodeshave an outside shell made of a steel sheet and an inside reinforcingstructure consisting of six fins welded radially to the inner wall ofthe outside shell. On the axis of each electrode there was arranged asteel strap 30 mm. wide and 2mm. thick.

Device 9 was a double moving element recording ohmmeter.

Conductors l0 and 11, suitably insulated and shielded, are connected tothe upper ends of the central bar 8 and of the metal shell 1 and to theinput terminals of the recording ohmmeter. The same meter 9 was used tomeasure the electrode current, suitably derived from a currenttransformer on the primary side of the current supply to the transformerof the furnace. This is not necessary to the invention and is not shownin FIG. 1 for simplicitys sake.

Ohmmeter 9 recorded the ratio between the two quantities introduced,that is, the potential between shell 1 and bar 9, and the electrodecurrent. The electrode current of the nor mal operation of the furnacewas 45,000 A. Voltage drops detected between shell 1 and bar 8 variedfrom about 0.2 to about 0.8 v., depending on the degree of baking of thecarbon mass layer 5, and on the electrode current.

In a constant electrode current run, wherein the current density was45,000 A., a 0.2 v. voltage drop was found to correspond to an excellentbaking degree and consequently the slipping of the electrode was madepossible.

For conveniences sake, the scale of the recording device was calibratedso that the ratio 0.2/45,000 was made to correspond to an index C valueof 1. Index C detected by the recording device, varied between about Iand about 2.3 depending on the baking degree of the electrode. Values ofindex C equal to or lower than 1 correspond to an excellent bakingdegree which allowed the slipping of the electrodes. Conversely, valuesof index C larger than 1, correspond to increasingly low baking degreesand have an increased risk of breakages. It is quite evident that therelationship between C=l and the ratio 0.2/45,000 is valid only for theparticular example given. Other relationships of shell-bar potential toelectrode current may be assigned the arbitrary C value of 1 when adifferent electrode structure and furnace are employed.

Thus, if the characteristics of the electrode (such as for instancediameter, electrode-fonning paste composition, shall and fins shape,current supplying conductors type, and so on) are changed, the index Cfigures corresponding respectively to excellent and poor bakingconditions, will also change. While operating at a fixed and constantelectrode current, after having determined the voltage drop figurecorresponding to the best baking conditions, the scale of the recordingdevice will be recalibrated and the value 1 of index C will be made tocorrespond to this new ratio Voltage drop corresponding to the bestbaking conditions Elec uqis 91 3595 9- In FIG. 2 the typical graph of Cis shown. The electrode was slipped down 2 cm. every hour withoutvarying the electrode current. Each slipping was clearly pointed out bya sudden increase of C which immediately dropped back to values equal toabout 1.5 times that preceding the slippage. Thereafter, in the courseof 1 hour, the value of c resumed its normal value 1 thereby indicatingthe baking of a portion of the electrode equal to the portion slippeddown.

FIG. 3 shows the graph of C during the period of time immediatelysubsequent to the time period graphed in FIG. 2. The electrode, afterthe repeated slippages shown in FIG. 2, was very long; slippage wastherefore discontinued for ll hours. During this period the value of Cdropped below unity, stabilizing itself around 0.8 and thus indicating abaking degree higher than the normal one.

Lastly, FIG. 4 illustrates a dangerous situation in which the electrodewas slipped 8 cm. at the 10th hour and again at the llth hour, beforethe value of C was restored to normal, and then the electrode was causedto be slipped 4 cm. Index C, after the usual fast peak, returned towardsthe figure 2.2, while the electrode, loaded at the normal current of45,000 A, demonstrated a serious risk of breakage which compelled theimmediate reduction of electrode current to 20,000 A., with aconsequential reduction in the output capacity of the furnace.

Variations can, of course, be made without departing from the spirit andscope of the invention.

Having thus described the invention, what is desired to be secured byLetters Patent and hereby claimed is:

l. A method for continuously measuring the baking degree of aself-baking electrode, having a current flowing therethrough, saidelectrode comprising a peripherally continuous vertically extendingshell, a carbon mass contained therein, said carbon mass comprising anupper layer of unbaked electrode-forming paste and a lower layer ofbaked electrode mass, said layers being in vertically stacked relation,said method comprising: providing a bar of electrically conductivematerial inside said carbon mass, said bar extending into both theunbaked electrode forming paste and the baked mass so as to enable anelectrical contact between said bar and said carbon mass:

measuring a voltage drop between said shell and said bar;

and

measuring a ratio between said voltage drop and said electrode currentin order to determine the baking degree of layer of baked electrodematerial, said layers being in a vertically stacked relation;

b. a bar made of an electrically conductive material disposed insidesaid carbon mass and extending into both the unbaked electrode-formingpaste and the baked material;

c. means for measuring a voltage drop between said bar and said shellcomprising means for connecting said voltage drop measuring means tosaid bar and said shell; and

d. means for measuring an electrode current.

3. The electrode structure of claim 2, wherein said bar is verticallypositioned.

4. The electrode structure of claim 3, wherein said bar is positionedalong the longitudinal axis of said electrode structure.

5. The electrode structure of claim 4, wherein said electricallyconductive material is a metal.

6. The electrode structure of claim 5, wherein said metal is consumablecontemporaneously with the carbon mass.

7. The electrode structure of claim 6, wherein said bar is shaped in theform of a flat strip.

8. A method in accordance with claim 1 wherein said measured ratio isinversely proportional to said baking degree, said baking degree beingdetermined in accordance with said ratio.

t i i i i

1. A method for continuously measuring the baking degree of aself-baking electrode, having a current flowing therethrough, saidelectrode comprising a peripherally continuous vertically extendingshell, a carbon mass contained therein, said carbon mass comprising anupper layer of unbaked electrode-forming paste and a lower layer ofbaked electrode mass, said layers being in vertically stacked relation,said method comprising: providing a bar of electrically conductivematerial inside said carbon mass, said bar extending into both theunbaked electrode forming paste and the baked mass so as to enable anelectrical contact between said bar and said carbon mass: measuring avoltage drop between said shell and said bar; and measuring a ratiobetween said voltage drop and said electrode current in order todetermine the baking degree of said electrode.
 2. A self-bakingelectrode structure comprising: a. a peripherally continuous verticallyextending shell for containing a carbon mass therewithin, a carbon masswithin said shell, said carbon mass comprising an upper layer of unbakedelectrode-forming paste and a lower layer of baked electrode material,said layers being in a vertically stacked relation; b. a bar made of anelectrically conductive material disposed inside said carbon mass andextending into both the unbaked electrode-forming paste and the bakedmaterial; c. means for measuring a voltage drop between said bar andsaid shell comprising means for connecting said voltage drop measuringmeans to said bar and said shell; and d. means for measuring anelectrode current.
 3. The electrode structure of claim 2, wherein saidbar is vertically positioned.
 4. The electrode structure of claim 3,wherein said bar is positioned along the longitudinal axis of saidelectrode structure.
 5. The electrode structure of claim 4, wherein saidelectrically conductive material is a metal.
 6. The electrode structureof claim 5, wherein said metal is consumable contemporaneously with thecarbon mass.
 7. The electrode structure of claim 6, wherein said bar isshaped in the form of a flat strip.
 8. A method in accordance with claim1 wherein said measured ratio is inversely proportional to said bakingdegree, said baking degree being determined in accordance with saidratio.